1. Field of the Invention
This invention relates to Christiansen cells and methods of forming color images having reduced color deterioration obtained with such cells.
2. Prior Art
A. Christiansen Cell Prior Art:
It is well known that, in general, a train of light waves changes direction, i.e., is refracted, when it crosses a boundary separating two media of different indices of refraction and that, except in special cases, only a portion of the incident light passes into the second medium, the remainder being reflected. The directions of propagation of both the reflected and transmitted waves are different from that of the incident wave.
These phenomena are exploited in the Christiansen filter which produces a narrow band pass of color and which has application in the infrared and ultraviolet as well as in the visible region. This filter was first described by C. Christiansen in 1884. The Christiansen filter consists of a finely divided transparent material suspended in an optically homogeneous medium. Constituents are chosen so that they have different but intersecting dispersion curves, i.e., the refractive indices are identical at a particular wave length .lambda..sub.c in or near the visible region, but differ for all other wave lengths. The filter is optically homogeneous for light of wave length .lambda..sub.c, i.e., such light is unaffected and passes through the filter without deviation or reflection. However, for all other wavelengths the filter is optically heterogeneous and such light is scattered as a result of the refraction and reflection which occur at the particle-medium interfaces. The degree of scattering for a given wavelength depends on the difference in the two refractive indices at that wavelength so that greater scattering is expected for wavelength farther from .lambda..sub.c. Consequently, the transmission curve for the unscatterd light exhibits a maximum at .lambda..sub.c.
The only disclosure of the formation of color images by the imagewise modulation of the Christiansen effect is contained in coassigned U.S. Pat. No. 3,951,520, issued Apr. 29, 1976, to Robert K. Waring. A Christiansen cell is formed with a mixture of two immiscible condensed phases having dispersions of refractive index different from each other but with indices of refraction matching at one wavelength in the vicinity of the visible region of the spectrum. The refractive index of one phase (or both) is then locally changed in accordance with a predetermined image so that the matching wavelength changes. Light is passed through the cell and the scattered and unscattered components of the light emerging from the cell are separated and either component is then focused to form a colored image. The use of polymerizable monomers is disclosed as one method of achieving imagewise modulation of the Christiansen effect with the various degrees of polymerization causing corresponding shifts in the dispersion curve of the continuous phase. The sharpness of images formed with such cells is gradually lost due to the combined diffusion of polymer formed and monomer across the image boundaries.
Stabilization of color images formed by photomodulation of the Christiansen effect is described in Applicant's coassigned application Ser. No. 650,800, filed Jan. 20, 1976, now U.S. Pat. No. 4,008,083. The stabilization results by using a photopolymerizable medium containing a polymerizable monomer and a cross-linking monomer in an amount sufficient to produce, upon exposure to actinic light, cross-linked polymer which stabilizes the color image formed and is optically homogeneous with any remaining medium. While the color image formed is essentially maintained over a period of time, there remains a desire to stabilize the color image further so that deterioration of color at the edges of the image is reduced and to form a flexible gel matrix throughout the Christensen cell to stabilize the unexposed material from possible undesirable flow.
Prior art which describes photopolymerization generally is set forth in the aforesaid application Ser. No. 650,800.
B. Epoxy Gelling Agents in Photosensitive Compositions
The use of epoxy compounds in photosensitive, hardenable compositions is described in Japanese Pat. No. 4,728,081, British Pat. No. 1,361,518 and Japanese Pat. No. 7,216,062. These references recognize photohardening characteristics of epxoy systems which contain polymerizable monomers. There is no consideration given to the effect of an epoxy compound on the imaging characteristics of the Christiansen effect or its use in forming a gel matrix to restrict fluid flow.